Apparatus and method for resetting a Z-axis sensor flux guide

ABSTRACT

A method and apparatus eliminate magnetic domain walls in a flux guide by applying, either simultaneously or sequentially, a current pulse along serially positioned reset lines to create a magnetic field along the flux guide, thereby removing the magnetic domain walls. By applying the current pulses in parallel and stepping through pairs of shorter reset lines segments via switches, less voltage is required.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of pending U.S. application Ser. No.13/406,149, filed Feb. 27, 2012, which is hereby incorporated herein byreference in its entirety.

FIELD

The exemplary embodiments described herein generally relate to the fieldof magnetoelectronic devices and more particularly to CMOS compatiblemagnetoelectronic field sensors used to sense magnetic fields.

BACKGROUND

Sensors are widely used in modern systems to measure or detect physicalparameters, such as position, motion, force, acceleration, temperature,pressure, etc. While a variety of different sensor types exist formeasuring these and other parameters, they all suffer from variouslimitations. For example, inexpensive low field sensors, such as thoseused in an electronic compass and other similar magnetic sensingapplications, generally are anisotropic magnetoresistance (AMR) baseddevices. In order to arrive at the required sensitivity and reasonableresistances that mesh well with CMOS, the sensing units of such sensorsare generally on the order of square millimeters in size. For mobileapplications, such AMR sensor configurations are too costly, in terms ofexpense, circuit area, and power consumption.

Other types of sensors, such as magnetic tunnel junction (MTJ) sensorsand giant magnetoresistance (GMR) sensors, have been used to providesmaller profile sensors, but such sensors have their own concerns, suchas inadequate sensitivity and being affected by temperature changes. Toaddress these concerns, MTJ, GMR, and AMR sensors have been employed ina Wheatstone bridge structure to increase sensitivity and to eliminatetemperature dependent resistance changes. For minimal sensor size andcost, MTJ or GMR elements are preferred. Typically, a Wheatstone bridgestructure uses magnetic shields to suppress the response of referenceelements within the bridge so that only the sense elements (and hencethe bridge) respond in a predetermined manner. However, the magneticshields are thick and their fabrication requires carefully tuned NiFeseed and plating steps. Another drawback associated with magneticshields arises when the shield retains a remnant field when exposed to astrong (˜5 kOe) magnetic field, since this remnant field can impair thelow field measuring capabilities of the bridge structure. To prevent theuse of magnetic shields and to sense an external magnetic in three axis(X, Y, Z), three Wheatstone bridge structures (one for each axis) areused. The layers of each bridge structure are fabricated in the sameprocesses in similar layers. In order to sense the magnetic field in theZ axis, flux guides are used to guide the Z axis field into the X-Yplane to be sensed by one of the bridge structures. These flux guideshave a preferred magnetization orientation for optimal Z axis response.Exposure to a very large external field in a particular orientation canreorient the flux guide magnetization so that upon returning to its lowfield sensing configuration, magnetic domain walls may be present in theZ axis flux guides. The tiny fluctuations in the dipolar field at thesense element generated by temperature induced motion of these domainwalls along the flux guide length can elevate the overall sensor noiseabove the lowest achievable output noise, and reduce signal to noiseratio (SNR).

Accordingly, a need exists for an improved design and fabricationprocess for forming a single chip magnetic sensor that is responsive anapplied magnetic field in three dimensions in which magnetic domainwalls in the Z axis flux guides may be eliminated, should the sensor beexposed to a large magnetic field. There is also a need for a three-axissensor that can be efficiently and inexpensively constructed as anintegrated circuit structure for use in mobile applications. There isalso a need for an improved magnetic field sensor and fabrication toovercome the problems in the art, such as outlined above. Furthermore,other desirable features and characteristics of the present inventionwill become apparent from the subsequent detailed description and theappended claims, taken in conjunction with the accompanying drawings andthis background.

BRIEF SUMMARY

A method and structure is/are provided for removing magnetic domains inZ-axis flux guides created by high external fields.

A first exemplary embodiment is a method of resetting flux guides in afield sensor, comprising applying a first field to first portion of aflux guide; applying a second field to a second portion of the fluxguide; applying a third field to a third portion of the flux guide; andapplying a fourth field to a fourth portion of the flux guide.

A second exemplary embodiment is a method of resetting a flux guide in afield sensor, the flux guide including n adjacent portions, wherein eachportion has one of a plurality of n reset lines positioned adjacentthereto, where n is an integer greater than 1, the method comprisingapplying one of n current pulses to each of the n reset lines.

A third exemplary embodiment is a field sensor including a plurality ofmagnetic field sensors, each sensor, comprising a sense element defininga plane; a flux guide configured to direct a magnetic fieldperpendicular to the plane into the plane, the flux guide including nadjacent portions of a continuous line, where n is an integer greaterthan 1; a plurality of n reset lines, each reset line positioneduniquely adjacent one of the n portions; and circuitry coupled to thereset lines and configured to apply a plurality of current pulses, oneeach to each of the n reset lines, thereby creating a field in each ofthe portions.

BRIEF DESCRIPTION OF THE DRAWINGS

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the drawings have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements for purposes of promoting andimproving clarity and understanding. Further, where consideredappropriate, reference numerals have been repeated among the drawings torepresent corresponding or analogous elements.

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 illustrates an electronic compass structure which usesdifferential sensors formed from three bridge structures with MTJsensors;

FIG. 2 is a view of flux lines as calculated by finite elementsimulation of two of the four magnetic tunnel junction sensors of FIG.1;

FIG. 3 is a partial cross section of the Z axis bridge structure of FIG.1 in accordance with a first exemplary embodiment;

FIG. 4 is a top view of the reset lines and sensors in accordance withthe first exemplary embodiment of FIG. 3;

FIG. 5 is a chart of switch settings versus time for the first exemplaryembodiment of FIG. 4;

FIG. 6 is a partial cross section of the Z axis bridge structure of FIG.1 in accordance with a second exemplary embodiment;

FIG. 7 is a top view of the reset lines and sensors in accordance withthe second exemplary embodiment;

FIG. 8 is a chart of switch settings versus time for the secondexemplary embodiment of FIG. 7;

FIG. 9 is a flow chart of a first exemplary method for the disclosedembodiments; and

FIG. 10 is a flow chart of a second exemplary method for the disclosedembodiments.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. Any implementation describedherein as exemplary is not necessarily to be construed as preferred oradvantageous over other implementations. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary, or thefollowing detailed description.

During the course of this description, like numbers are used to identifylike elements according to the different figures that illustrate thevarious exemplary embodiments.

Through the integration of high aspect ratio vertical bars (flux guides)of a high permeability material, for example, nickel iron (NiFe), whoseends terminate in close proximity to opposed edges and/or opposite sidesof a magnetic sense element, a portion of the Z axis field can be guidedinto the XY plane (see FIGS. 1 and 2). These flux guides serve tocapture magnetic flux from an applied field oriented in the Z direction,and in so doing, bend the field lines in a substantially horizontalmanner near the ends of the flux guides. Through asymmetric positioningof the flux guides, e.g., the flux guide segment above the left edge ofsense elements in two legs of the four legs of a Wheatstone bridge, andthe flux guide above the right edge of sense elements in the other twolegs, the horizontal components may act in an opposite directions forthe two pairs of legs resulting in a strong differential signal. A fieldapplied in the X or Y direction will project equally on all four legs ofthe bridge and hence be subtracted out and not contribute to the finalsensor signal. Separate bridges are included elsewhere on the magneticsensor chip for determining the X and Y components of the magneticsignal, and in this manner, a field with components in all three spatialorientations can be accurately determined by a single chipmagnetoresistive sensing module, for example, based on magnetic tunneljunction (MTJ) sense elements. Finite Element Method (FEM) simulationshave shown that a pair of high aspect ratio flux guides, e.g., 25 nmwide by 500 nm high and extending several microns in the thirddirection, when optimally positioned will provide a signal on anindividual element that is about 80% of the of the signal measured froman in plane (x axis) field of the same strength. Additional signal maybe obtained through closer proximity of the flux guide to the sensor,increases in the flux guide height, and additional shaping of the guidegeometry. These geometries serve to further enhance the horizontalcomponent of the guided flux and move it to a more central region of thesensor. A structure with individual 25 nm wide vertical bars utilized asflux guides is tolerant to overlay errors and produces an apparent x toz field conversion (for a differentially wired Wheatstone bridge) at therate of 2.5% for a misalignment of 85 nm (3 sigma) between a single fluxguiding layer and the sense layer.

The flux guiding layer may be formed from layers typically used in themagnetic random access memory (MRAM) process flow, during which bit anddigit lines cladded with a high permeability magnetic material (such asin typical magnetic memory devices), referred to herein as a flux guide,are used to increase the field factors present to reduce the currentneeded to switch the memory storage element. In the sensor application,the same process flow may be used with the optional additional step ofsputtering out the bottom of the digit line in order to remove anycladding present on the trench's bottom. Modifications may be made tothe process flow so that the height and width of the cladding used forflux guiding are at optimum values instead of the 500 nm and 25 nm,respectively that are used in the exemplary process described above.

A drawback to using flux guides in a Z axis to direct the flux into theX-Y plane of the Z field sensor in the X-Y plane of the Wheatstonebridge is the formation of magnetic domain walls in the flux guide.These magnetic domain walls create noise that may lower the SNR duringmeasurements of an external field. The exemplary embodiments describedherein eliminate these magnetic domain walls.

A method and apparatus are described in more detail for eliminatingmagnetic domain walls in a flux guide by applying a plurality of currentpulses through a plurality of parallel reset lines or sequentiallyapplying a current pulse along serially positioned reset lines to createa magnetic field sequentially progress along the length of the fluxguide. The reset lines may be on one side of the flux guides with thecurrent pulse directed in a first direction, or the reset lines may bepositioned on opposed sides of the flux guides with the current pulsedirected in opposed directions on the opposed sides. As the magneticfield generated by the sequentially applied current pulses progressesalong the flux guide, the magnetic domains are swept along the length ofthe guide and through the end, thereby removing the magnetic domainwalls in the process. By applying the current pulses along the series ofshorter reset lines, less voltage is required over that required bypassing the current pulse through one entire reset line.

More specifically, the reset/stabilization line is divided into multiplesegments. These segments are connected in parallel which enables theflow of a relatively high current required by reset field at a givensupply voltage. The reset current through these line segments can beapplied simultaneously or sequentially with some spatial and temporaloverlap by the introduction of the switches in series with the segmentedreset/stabilization line as well as switches that directly connectvarious sections of the reset line either to power or to ground. Thoseswitches enable the application of the reset current pulses through eachsegment or a subset of segments in parallel (when these switches areopen), simultaneously or sequentially with overlap, and stabilizationcurrent serially (when these switches are closed).

Various illustrative embodiments of the present invention will now bedescribed in detail with reference to the accompanying figures. Whilevarious details are set forth in the following description, it will beappreciated that the present invention may be practiced without thesespecific details, and that numerous implementation-specific decisionsmay be made to the invention described herein to achieve the devicedesigner's specific goals, such as compliance with process technology ordesign-related constraints, which will vary from one implementation toanother. While such a development effort might be complex andtime-consuming, it would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure. Inaddition, selected aspects are depicted with reference to simplifiedcross sectional drawings without including every device feature orgeometry in order to avoid limiting or obscuring the present invention.It is also noted that, throughout this detailed description,conventional techniques and features related to magnetic sensor designand operation, Magnetoresistive Random Access Memory (MRAM) design, MRAMoperation, semiconductor device fabrication, and other aspects of theintegrated circuit devices may not be described in detail herein. Whilecertain materials will be formed and removed to fabricate the integratedcircuit sensors as part of an existing MRAM fabrication process, thespecific procedures for forming or removing such materials are notdetailed below since such details are well known and not considerednecessary to teach one skilled in the art of how to make or use thepresent invention. Furthermore, the circuit/component layouts andconfigurations shown in the various figures contained herein areintended to represent exemplary embodiments of the invention. It shouldbe noted that many alternative or additional circuit/component layoutsmay be present in a practical embodiment.

FIG. 1 shows a magnetic field sensor 100 (as described in U.S. patentapplication Ser. No. 13/031,558 assigned to the Assignee of this presentapplication) formed with first, second, and third differential sensors101, 111, 121 for detecting the component directions of an applied fieldalong a first axis 120 (e.g., the y-axis direction), a second axis 110(e.g., the x-axis direction), and a third axis 130 (e.g., the z-axisdirection), respectively. The z-axis direction is represented as a dotand cross-hairs as going either into or out of the page on which FIG. 1is situated. Exemplary embodiments of the first and second sensors 101,111 are described in detail in U.S. patent application Ser. No.12/433,679. As depicted herein, each sensor 101, 111, 121 is formed withunshielded sense elements that are connected in a bridge configuration.Thus, the first sensor 101 is formed from the connection of a pluralityof sense elements 102-105 in a bridge configuration over a correspondingplurality of pinned layers 106-109, where each of the pinned layers106-109 is magnetized in the x-axis direction. In similar fashion, thesecond sensor 111 is formed from the connection of a plurality of senseelements 112-115 in a bridge configuration over a correspondingplurality of pinned layers 116-119 that are each magnetized in they-axis direction that is perpendicular to the magnetization direction ofthe pinned layers 106-109. Furthermore, the third sensor 121 in the sameplane as the first and second sensors 101, 111 is formed from theconnection of a plurality of sense elements 122-125 in a bridgeconfiguration over a corresponding plurality of pinned layers 126-129that are each magnetized along either the xy-axis direction to themagnetization direction of the pinned layers 106-109 and 116-119. In thedepicted bridge configuration 101, the sense elements 102, 104 areformed to have a first easy axis magnetization direction and the senseelements 103, 105 are formed to have a second easy axis magnetizationdirection, where the first and second easy axis magnetization directionsare orthogonal with respect to one another and are oriented to differequally from the magnetization direction of the pinned layers 106-109.As for the second bridge configuration 111, the sense elements 112, 114have a first easy axis magnetization direction that is orthogonal to thesecond easy axis magnetization direction for the sense elements 113, 115so that the first and second easy axis magnetization directions areoriented to differ equally from the magnetization direction of thepinned layers 116-119. In the third bridge configuration 121, the senseelements 122 123,124, and 125 all have an easy axis magnetizationdirection that is orthogonal to the pinned magnetization direction ofthe pinned layers 126, 127, 128, and 129. The third bridge configuration121 further includes flux guides 132-135 positioned adjacent to theright edge of sense elements 122-125, and flux guides 136-139 positionedadjacent to the left edge of sense elements 122-125, respectively. Fluxguides 132,137, 134, and 139 are positioned above sense elements122-125, and flux guides 136, 133, 138, and 135 are positioned belowsense elements 122-125. In the depicted sensors 101, 111, 121 there isno shielding required for the sense elements, nor are any specialreference elements required. In an exemplary embodiment, this isachieved by referencing each active sense element (e.g., 102, 104) withanother active sense element (e.g., 103, 105) using shape anisotropytechniques to establish the easy magnetic axes of the referenced senseelements to be deflected from each other by 90 degrees for the x and ysensors, and referencing a sense element that responds in an oppositemanner to an applied field in the Z direction for the Z sensor. The Zsensor referencing will be described in more detail below. Theconfiguration shown in FIG. 1 is not required to harvest the benefits ofthe third sensor 121 structure generally described in FIG. 2, and isonly given as an example.

By positioning the first and second sensors 101, 111 to be orthogonallyaligned, each with the sense element orientations deflected equally fromthe sensor's pinning direction and orthogonal to one another in eachsensor, the sensors can detect the component directions of an appliedfield along the first and second axes. Flux guides 132,133,136,137 arepositioned in sensor 121 above and below the opposite edges of theelements 122-123, in an asymmetrical manner between legs 141 and 142. Asflux guide, 137 is placed above sense element 123, the magnetic fluxfrom the Z field may be guided by the flux guides 137 and 133 into thexy plane along the right side and cause the magnetization of senseelement 123 to rotate in a first direction towards a higher resistance.Similarly, the magnetic flux from the Z field may be guided by the fluxguides 132 and 136 into the xy plane along the left side of the senseelement and cause the magnetization of sense element 122 to rotate in asecond direction, opposite from the first direction towards a lowerresistance as these guides are antisymmetric to guides 137, 133 . . . .Thus, the sensor 121 can detect the component directions of an appliedfield along the third axis. Although in the preferred embodiment, theflux guides are in a plane orthogonal to the plane of the field sensor,the flux guides will still function if the angle they make with thesensor is not exactly 90 degrees. In other embodiments, the anglebetween the flux guide and the field sensor could be in a range from 45degrees to 135 degrees, with the exact angle chosen depending on otherfactors such as on the ease of fabrication.

As seen from the foregoing, a magnetic field sensor may be formed fromdifferential sensors 101, 111, 121 which use unshielded sense elements102-105, 112-115, and sense elements 122-125 with guided magnetic fluxconnected in a bridge configuration over respective pinned, orreference, layers 106-109, 116-119, and 126-129 to detect the presenceand direction of an applied magnetic field. With this configuration, themagnetic field sensor provides good sensitivity, and also provides thetemperature compensating properties of a bridge configuration.

The bridge circuits 101, 111, 121 may be manufactured as part of anexisting MRAM or thin-film sensor manufacturing process with only minoradjustments to control the magnetic orientation of the various sensorlayers and cross section of the flux guiding structures. Each of thepinned layers 106-109, 116-119, and 126-129 may be formed with one ormore lower ferromagnetic layers, and each of the sense elements 102-105,112-125, 122-125 may be formed with one or more upper ferromagneticlayers. An insulating tunneling dielectric layer (not shown) may bedisposed between the sense elements 102-105, 112-125, 122-125 and thepinned layers 106-109, 116-119, and 126-129. The pinned and senseelectrodes are desirably magnetic materials whose magnetizationdirection can be aligned. Suitable electrode materials and arrangementsof the materials into structures commonly used for electrodes ofmagnetoresistive random access memory (MRAM) devices and other magnetictunnel junction (MTJ) sensor devices are well known in the art. Forexample, pinned layers 106-109, 116-119, and 126-129 may be formed withone or more layers of ferromagnetic and antiferromagnetic materials to acombined thickness in the range 10 to 1000 Å, and in selectedembodiments in the range 200 to 350 Å. In an exemplary implementation,each of the pinned layers 106-109, 116-119, and 126-129 is formed with asingle ferromagnetic layer and an underlying anti-ferromagnetic pinninglayer. In another exemplary implementation, each pinned layer 106-109,116-119, and 126-129 includes a synthetic anti-ferromagnetic stackcomponent (e.g., a stack of CF (Cobalt Iron), Ruthenium (Ru), and CFB(Cobalt Iron Boron)) which is 20 to 80 Å thick, and an underlyinganti-ferromagnetic pinning layer that is approximately 200 Å thick. Thelower anti-ferromagnetic pinning materials may be re-settable materials,such as IrMn, though other materials, such as PtMn, can be used whichare not readily re-set at reasonable temperatures. As formed, the pinnedlayers 106-109, 116-119, and 126-129 function as a fixed or pinnedmagnetic layer when the direction of its magnetization is pinned in onedirection that does not change during normal operating conditions. Asdisclosed herein, the heating qualities of the materials used to pin thepinned layers 106-109, 116-119, and 126-129 can change the fabricationsequence used to form these layers.

One of each of the sense elements 102-105, 112-125, 122-125 and one ofeach of the pinned layers 106-109, 116-119, 126-129 form a magnetictunnel junction (MTJ) sensor. For example, for bridge circuit 121, senseelement 122 and pinned layer 126 form an MTJ sensor 141. Likewise, senseelement 123 and pinned layer 127 form an MTJ sensor 142, sense element124 and pinned layer 128 form an MTJ sensor 143, and sense element 125and pinned layer 129 form an MTJ sensor 144.

The pinned layers 106-109, 116-119, and 126-129 may be formed with asingle patterned ferromagnetic layer having a magnetization direction(indicated by the arrow) that aligns along the long-axis of thepatterned reference layer(s). However, in other embodiments, the pinnedreference layer may be implemented with a synthetic anti-ferromagnetic(SAF) layer which is used to align the magnetization of the pinnedreference layer along the short axis of the patterned referencelayer(s). As will be appreciated, the SAF layer may be implemented incombination with an underlying anti-ferromagnetic pinning layer, thoughwith SAF structures with appropriate geometry and materials that providesufficiently strong magnetization, the underlying anti-ferromagneticpinning layer may not be required, thereby providing a simplerfabrication process with cost savings.

The sense elements 102-105, 112-125, 122-125 may be formed with one ormore layers of ferromagnetic materials to a thickness in the range 10 to5000 Å, and in selected embodiments in the range 10 to 60 Å. The upperferromagnetic materials may be magnetically soft materials, such asNiFe, CoFe, Fe, CFB and the like. In each MTJ sensor, the sense elements102-105, 112-125, 122-125 function as a sense layer or free magneticlayer because the direction of their magnetization can be deflected bythe presence of an external applied field, such as the Earth's magneticfield. As finally formed, sense elements 102-105, 112-125, 122-125 maybe formed with a single ferromagnetic layer having a magnetizationdirection (indicated with the arrows) that aligns along the long-axis ofthe patterned shapes.

The pinned layers 106-109, 116-119, 126-129 and sense elements 102-105,112-125, 122-125 may be formed to have different magnetic properties.For example, the pinned layers 106-109, 116-119, 126-129 may be formedwith an anti-ferromagnetic film exchange layer coupled to aferromagnetic film to form layers with a high coercive force and offsethysteresis curves so that their magnetization direction will be pinnedin one direction, and hence substantially unaffected by an externallyapplied magnetic field. In contrast, the sense elements 102-105,112-125, 122-125 may be formed with a magnetically soft material toprovide different magnetization directions having a comparatively lowanisotropy and coercive force so that the magnetization direction of thesense electrode may be altered by an externally applied magnetic field.In selected embodiments, the strength of the pinning field is about30-100× larger than the anisotropy field of the sense electrodes,although different ratios may be used by adjusting the respectivemagnetic properties of the electrodes using well known techniques tovary their composition.

The pinned layers 106-109, 116-119, 126-129 in the MTJ sensors areformed to have a shape determined magnetization direction in the planeof the pinned layers 106-109, 116-119, 126-129 (identified by the vectorarrows for each sensor bridge labeled “Pinning direction” in FIG. 1). Asdescribed herein, the magnetization direction for the pinned layers106-109, 116-119, 126-129 may be obtained using shape anisotropy of thepinned electrodes, in which case the shapes of the pinned layers106-109, 116-119, 126-129 may each be longer in the pinning directionfor a single pinned layer. Alternatively, for a pinned SAF structure,the reference and pinned layers may be shorter along the pinningdirection. In particular, the magnetization direction for the pinnedlayers 106-109, 116-119, 126-129 may be obtained by first heating theshaped pinned layers 106-109, 116-119, 126-129 in the presence of aorienting magnetic field which is oriented non-orthogonally to the axisof longest orientation for the shaped pinned layers 106-109, 116-119,126-129 such that the applied orienting field includes a field componentin the direction of the desired pinning direction for the pinned layers106-109, 116-119, 126-129. The magnetization directions of the pinnedlayers are aligned, at least temporarily, in a predetermined direction.However, by appropriately heating the pinned layers during thistreatment and removing the orienting field without reducing the heat,the magnetization of the pinned layers relaxes along the desired axis oforientation for the shaped pinned pinned layers 106-109, 116-119,126-129. Once the magnetization relaxes, the pinned layers can beannealed above the recrystallization temperature to set theantiferromagnet (in the case of PtMn) or cooled below the blockingtemperature (in the case of IrMn) so that the magnetic field directionof the pinned electrode layers is set in the desired direction for theshaped pinned layers 106-109, 116-119, 126-129.

The exemplary embodiments described herein may be fabricated using knownlithographic processes as follows. The fabrication of integratedcircuits, microelectronic devices, micro electro mechanical devices,microfluidic devices, and photonic devices involves the creation ofseveral layers of materials that interact in some fashion. One or moreof these layers may be patterned so various regions of the layer havedifferent electrical or other characteristics, which may beinterconnected within the layer or to other layers to create electricalcomponents and circuits. These regions may be created by selectivelyintroducing or removing various materials. The patterns that define suchregions are often created by lithographic processes. For example, alayer of photoresist material is applied onto a layer overlying a wafersubstrate. A photomask (containing clear and opaque areas) is used toselectively expose this photoresist material by a form of radiation,such as ultraviolet light, electrons, or x-rays. Either the photoresistmaterial exposed to the radiation, or that not exposed to the radiation,is removed by the application of a developer. An etch may then beapplied to the layer not protected by the remaining resist, and when theresist is removed, the layer overlying the substrate is patterned.Alternatively, an additive process could also be used, e.g., building astructure using the photoresist as a template.

FIG. 2 is a view of flux lines as calculated by finite elementsimulation of MTJ devices 141, 142 of FIG. 1 with a magnetic field inthe z direction imparted upon the sense elements 122-123. FEM modelingshows the resultant magnetic flux lines 160, exhibiting a component inthe plane of the sensor. MTJ device 141 is represented by flux guides132 and 136 on opposed ends of the sensing element 122. MTJ device 142is represented by flux guides 133 and 137 on opposed ends of the sensingelement 123. Stated otherwise, sensing element 122 extends from fluxguides 132 and 136, and sensing element 123 extends from flux guides 133and 137. The magnetic field 160 in the Z-axis 130 produces an asymmetricresponse in the sensing elements 122, 123 along the X-axis 120 asindicated by the arrows 170. In this manner, for a field 160 in the Zdirection 130 directed towards the bottom of the page, the magnetizationof sense element 122 rotates away from the pinning direction (and tohigher resistance) of the pinned layer 126, while the magnetization ofsense element 123 rotates towards the pinning direction (and to lowerresistance) of pinned layer 127. For a field in the X direction 120,both elements 122, 123 show induced magnetization in the same direction(towards higher or lower resistance). Therefore, by wiring MTJ elements141, 142 in a Wheatstone bridge for differential measurement andsubtracting the resistances of MTJ devices 141, 142, the X fieldresponse is eliminated and twice the Z field response is measured.

However, a strong magnetic field 160 will cause the magnetization of theflux guide to wind up completely out of plane of the sensor substrate.When the magnetization relaxes from this high field state, the chiralityof the relaxation path may differ in adjacent regions of a single fluxguide, resulting in local magnetization that points in oppositedirections along the length of the flux guide. This will create magneticdomain walls in the flux guides 132, 137. The domain walls maysubsequently travel up and down the length of the flux guide due to lowlevel thermal excitations, thereby modulating the local field at thesense elements, and hence elevate the noise floor above the lowestpossible minimum. A subsequent measurement of a low external field willthen take place over the elevated noise floor, degrading the overallsensor SNR. The exemplary embodiments described herein eliminate thesemagnetic domain walls.

Referring to FIG. 3 and in accordance with an exemplary embodiment ofthe present invention, a partial cross section of the MTJ devices 141 ofthe third bridge circuit 121 include the sense element 122, and the fluxguide 136, all formed within the dielectric material 140. The flux guide136 has an end positioned below an edge of the sensor element 122. Ametal stabilization/reset line 152 is positioned on one side of the MTJdevice 141, and orthogonal to the flux guide 136, for providing a largecurrent pulse 154 in a direction 154, for example, that creates a resetfield 130 acting upon both the sense elements and the flux guide 136.The reset field 130 is represented by a dot indicating its direction iscoming from the page. Alternatively, the metal line 152 may be onopposed sides (not shown) of the MTJ device 141 with the current pulseflowing in opposed directions. The same line 152 can also be used asstabilization line. A current flowing through 152 creates astabilization field at the sense elements 122. The ends of the fluxguides may be brought as close as possible to the sensor elements, witha preferable spacing of less than or equal to 500 nm between the two.

A top view of the structure of FIG. 3 is shown in FIG. 4 and includesswitches 151, 153, 155, 157 coupling a voltage from conductor 160through the reset lines 152, 154, 156, 158 to ground at conductor 161. Aplurality of reset lines 152, 154, 156, 158 is positioned below, andorthogonal to, the flux guide 136. In another embodiment, the reset linecan be placed above or on both sides of the flux guide and the senseelements.

In operation, the switches 151, 153, 155, 157 are “closed” in a sequenceto provide a current pulse in the reset lines 152, 154, 156, 158 thatcreates the magnetic field 130 from one end 159 of the flux guide 136 tothe other end 166 of the flux guide 136. An exemplary sequence for theclosing of the switches 151, 153, 155, 157 is illustrated in the chartof FIG. 5 wherein a “1” signifies the switch is closed and a “0”signifies the switch is open. During time period t1, switch 151 isclosed, creating a magnetic field in the portion 162 of the flux guide136. The magnetic domain walls in the portion 162, if any, may be movedtowards the other end 166 of the flux guide 136 by this magnetic field.However, if the current were removed from the reset line 152, themagnetic domain walls may come back and be reestablished in portion 162.In order to prevent this reestablishment of the magnetic domain, switch151, as well as switch 153, are closed during time t2. Then during timet3, switches 153 and 155 are closed. In time t4, switches 155 and 157are closed, and finally, during time t5, switch 157 is closed. Thissequence of “sweeping” the magnetic reset field along the flux guide 136sweeps the magnetic domain walls from one end to the other end of theflux guide, preventing the magnetic domains from being reestablished inthe portions 162-165 after the reset field has been removed. At the endof the sequence, the entire flux guide is in a single domain state. Byapplying the reset current in sequence along series of reset lines 152,154, 156, 158 instead of simultaneously along one long reset line, theindividual reset line segment resistance can be significantly reduced.As a result, the magnitude of the reset current pulse is drasticallyincreased given a certain reset bias voltage. The higher reset currentpulse increases the reset field strength and therefore ability to removethe magnetic domain walls in the flux guide.

During measurement, a lower current pulse may be applied simultaneouslythrough the switches 151, 153, 155, 157 to stabilize the sensor 122.

Another exemplary embodiment for reducing magnetic domain walls from theflux guide 136 is described with reference to FIGS. 7, 8, 9. A partialcross section (FIG. 6) of the MTJ devices 141 of the third bridgecircuit 121 includes the sense element 122, and the flux guide 136. Theflux guide 136 has an end positioned below an edge of the sensor element122. A metal stabilization/reset line 702 is positioned on opposed sidesof the MTJ device 141, and orthogonal to the flux guide 136, forproviding a large current pulse in a direction (indicated by arrow 710),for example, that creates a reset field 130 acting on the sense elementsand a reset field on the flux guide 136. The same line 702 can also beused as stabilization line. A current flowing through 702 creates astabilization field on the sense elements 122. The stabilization/resetfield 130 is represented by a dot indicating its direction is comingfrom the page. The ends of the flux guides may be brought as close aspossible to the sensor elements, with a preferable spacing of less thanor equal to 500 nm between the two.

A top view of the structure of FIG. 6 is shown in FIG. 7 and includesswitch pairs 711, 712; 713, 714; 715, 716; and 717, 718. Instabilization mode, the switches are configured to couple a voltage fromconductor 719 through the reset line segments 702, 704, 706, 708,respectively, to ground at conductor 718. Therefore, switches 711, 721,724, 726, 718 are closed, while the rest are open. The plurality ofreset lines 702, 704, 706, 708 are positioned orthogonal to the fluxguide 136, and pass over the sense elements 122 in a first direction andunder the sense elements 122 in a second direction. While the resetlines 702, 704, 706, 708 are offset for ease of description, theypreferably are aligned with the center of the sense elements 122.

In flux guide reset operation, switches 721, 724, 726 are opened and theswitch pairs 711, 712; 713, 714; 715, 716; and 717, 718 are “closed” ina sequence to provide current pulses in the reset line segments 702,704, 706, 708 that creates the magnetic field 130 that sweeps insequence from one end 732 of the flux guide 136 to the other end 738 ofthe flux guide 136. An exemplary sequence for the closing of the switchpairs 711, 712; 713, 714; 715, 716; and 717, 718 is illustrated in thechart of FIG. 8 wherein a “1” signifies the switch is closed and a “0”signifies the switch is open. During time period t1, switch pair 711 and712 are closed, creating a magnetic field in the portion 732 of the fluxguide 136. A magnetic domain wall in the portion 732, if present, may bemoved towards the other end 738 of the flux guide 136 under thismagnetic field. However, if the current were removed from the reset line136, a magnetic domain wall may come back and be reestablished inportion 732. In order to prevent this reestablishment of the magneticdomain, switches 713 and 714, as well as switches 711 and 712, areclosed during time t1. Then during time t2, switches 711 and 712 areopened, 713 and 714 stay closed, and switches 715 and 716 are closed. Intime t4, switches 713 and 714 are opened, 715 and 716 stay closed, andswitches 717 and 718 and are closed. This sequence of “walking” themagnetic reset field along the flux guide 136 sweeps the magnetic domainwalls from one end of the flux guide to the other while preventing themagnetic domains from reestablishing in the portions wherein themagnetic domains had previously been removed after the reset field hasbeen removed. By applying the reset current in a series of portions ofthe reset line 136 instead of along the entire reset line, the resetline resistance can be significantly reduced. As a result, the magnitudeof the reset current pulse is drastically increased given a certainreset bias voltage. The higher reset current pulse increases the resetfield strength and therefore ability to remove the magnetic domain wallsin the flux guide.

Note that the flux guide reset line segments 732, 734, 736, 738 extendall the way to the ends of the flux guide 136 in order to completelyclear any domain structure from the entire length of flux guide 136. Ifthe same number of reset segments 732, 734, 736, 738 is used as thoserequired to reset all the sense elements 122 in the array (see patentapplication Ser. No. 13/031,558, filed 21 Feb. 2011 by the Assignee ofthe present application), local fields may not be strong enough to cleardomains from the end of the flux guide 136 as it necessarily extendsbeyond the final sense element in the array. In this manner, domains arenot “stuck” near the ends of the flux guide 136. Otherwise after highfield exposure (domain creation) and the subsequent flux guide reset, anexposure to a low field in the right direction can cause a domain wallnear the end to travel back towards a sense element 122 and disrupt itslocal field response, resulting in noise at the sensor output. This isshown in FIG. 7 as the final reset line segments 702, 708 overly theends of the flux guide 136, and no sense elements are present at theflux guide ends. As the ends of the flux guide 136 are pointed to trapdomains, and prevent their nucleation at low and moderate fields, ahigher local field is likely required to reverse domain structure at theend of the flux guide 136. This could be done by 1) increasing thevoltage source at switch 717, 2) lengthening the width of the finalpulse (increasing the time that switches 717 and 718 are closed at theend of the pulse train), or 3) increasing the flux reset line segmentspatial density right at the flux guide end; i.e. segment 708 could bewider to allow a higher current to flow or could have several identicalsegments that are very close to one another, switched on at the sametime. These temporal or intensity adjustments may be made at both endsof the flux guide, but are most important to incorporate at the finalend to receive the reset pulse.

While the flux guide reset is preferably configured to reset all thesense elements 122, it may be required to insert additional reset linesegments (not shown) between sense elements 122; i.e. covering thelength between flux guide segments 734 and 736 to keep the domain wallstraveling along length of the flux guide 136. This requirement dependsupon the ease with which domains travel (field required to continuemotion) along the length of the flux guide 136, which will depend uponmaterial characteristics of the flux guides 136 as well as geometricalproperties such as side wall roughness. As such, the flux guidesmaterials should be selected from materials with a relatively highdomain wall velocity for shorter pulse width, e.g., greater than 100 m/sfor pulses of less than 100 ns, and the side walls should be as smoothas possible, to reduce the required pulse amplitude. Proper materialselection and processing conditions will enable a lower density of resetline segments. Also, the end taper rules (which prefer AR>=2.5 for highdomain wall nucleation threshold) may be relaxed to lower the fieldrequired at the flux guide ends. Because domain wall motion is slowerthan the sense element reversal time, it is highly beneficial to createtwo reset modes. The first reset mode is for the sense elements only,and may proceed with ns pulse width time scales, and a lower total pulsecount (since it is not necessary to continue the pulse train all the wayto the end of the flux guide). This mode will be required morefrequently as a moderate field exposure is more commonplace, and couldbe incorporated to precede every measurement. The second reset modewould be to reset or clear the flux guide domain structure. Due to thedomain wall velocity limitations, this would require pulse widths on theorder of 1 μsec and require more pulses for a higher total current orpower dissipation. However, this mode would be required much lessfrequently as the much larger field exposures required to create multidomain structures are quite infrequent. An algorithm to determine whichreset to apply includes measuring the noise in the sensor output,applying a sense element reset if it is higher than a threshold,measuring the noise again, and applying a flux guide reset if it isstill higher than the threshold. It may also be beneficial to apply aflux guide reset at sensor startup, to ensure that the flux guides areproperly initialized. Also, reading from accelerometers and the in planesensors may be utilized to predict what the Z component of the measuredmagnetic field should be based upon GPS location. If the measured Zcomponent differs significantly from this predicted value, a flux guidereset may be applied.

During measurement, a lower current pulse may be applied simultaneouslythrough the switches 711, 721, 724, 726 and 718 to stabilize the sensors122.

FIGS. 9 and 10 are flow charts that illustrate exemplary embodiments ofa process 900 and 1000 suitable for removing magnetic domains from the Zaxis flux guide 136. It should be appreciated that processes 900 and1000 may include any number of additional or alternative tasks, thetasks shown in FIGS. 9 and 10 need not be performed in the illustratedorder, and processes 900 and 1000 may be incorporated into a morecomprehensive procedure or process having additional functionality notdescribed in detail herein. Moreover, one or more of the tasks shown inFIGS. 9 and 10 could be omitted from an embodiment of the processes 900and 1000 as long as the intended overall functionality remains intact.

Referring to FIG. 9, the process 900 includes sequentially applying 902a first field to a first portions of a flux guide, applying 904 a secondfield to a second portion of the flux guide, applying 906 a third fieldto a third portion of the flux guide, and applying 908 a fourth field toa fourth portion of the flux guide.

The process 1000 of FIG. 10 includes applying one of N current pulses toeach one of N reset lines adjacent each of N portions of a flux guide,where N is an integer greater than 1.

Although the described exemplary embodiments disclosed herein aredirected to various sensor structures and methods for making same, thepresent invention is not necessarily limited to the exemplaryembodiments which illustrate inventive aspects of the present inventionthat are applicable to a wide variety of semiconductor processes and/ordevices. Thus, the particular embodiments disclosed above areillustrative only and should not be taken as limitations upon thepresent invention, as the invention may be modified and practiced indifferent but equivalent manners apparent to those skilled in the arthaving the benefit of the teachings herein. For example, the relativepositions of the sense and pinning layers in a sensor structure may bereversed so that the pinning layer is on top and the sense layer isbelow. Also the sense layers and the pinning layers may be formed withdifferent materials than those disclosed. Moreover, the thickness of thedescribed layers may deviate from the disclosed thickness values.Accordingly, the foregoing description is not intended to limit theinvention to the particular form set forth, but on the contrary, isintended to cover such alternatives, modifications and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims so that those skilled in the art shouldunderstand that they can make various changes, substitutions andalterations without departing from the spirit and scope of the inventionin its broadest form.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing an exemplary embodiment of the invention, it beingunderstood that various changes may be made in the function andarrangement of elements described in an exemplary embodiment withoutdeparting from the scope of the invention as set forth in the appendedclaims.

What is claimed is:
 1. A method of operating a plurality of magneticfield sensors of a field sensor, the method comprising: sensing, by afield sensor including a plurality of magnetic field sensors, a magneticresponse in a direction out of a plane defined by the plurality ofmagnetic field sensors of the field sensor, each magnetic field sensorincluding a sensing element having a first side and a second side, aflux guide disposed orthogonal to and spaced from the first side of thesensing element, and a plurality of reset lines positioned proximate thesensing element and adjacent to the flux guide, wherein the field sensorfurther includes control circuitry operably connected to each reset lineand configured to selectively apply at least one current pulse to eachreset line of the plurality of reset lines; selectively applying aplurality of current pulses to the plurality of reset lines; andgenerating a reset field to re-orient a magnetic domain in the fluxguide, wherein the reset field is generated as a result of theselectively applied plurality of current pulses.
 2. The method of claim1, wherein selectively applying the plurality of current pulses to theplurality of reset lines includes: applying a plurality of currentpulses simultaneously to the plurality of reset lines.
 3. The method ofclaim 1, wherein selectively applying the plurality of current pulses tothe plurality of reset lines includes: applying a plurality of currentpulses sequentially to the plurality of reset lines.
 4. The method ofclaim 1, wherein selectively applying the plurality of current pulses tothe plurality of reset lines includes: applying a first current pulse toa first reset line of the plurality of reset lines; removing the firstcurrent pulse to the first reset line of the plurality of reset lines;and applying a second current pulse to a second reset line of theplurality of reset lines after removing the first current pulse to thefirst reset line.
 5. The method of claim 1, wherein selectively applyingthe plurality of current pulses to the plurality of reset linesincludes: applying a first current pulse to a first reset line of theplurality of reset lines; applying a second current pulse to a secondreset line of the plurality of reset lines, wherein the second resetline is immediately adjacent the first reset line; and removing thefirst current pulse to the first reset line, wherein the first currentpulse is removed after the second current pulse is applied to the secondreset line.
 6. The method of claim 1, wherein selectively applying theplurality of current pulses to the plurality of reset lines includes:applying a current pulse simultaneously to each of the plurality ofreset lines so as to stabilize the field sensor during operation.
 7. Themethod of claim 1, further comprising: determining a noise of the fieldsensor based on the sensed magnetic response, wherein selectivelyapplying the plurality of current pulses to the plurality of reset linesincludes: applying a current pulse to a reset line of the plurality ofreset lines when the noise is higher than a predetermined threshold. 8.A magnetic field sensor, comprising: a sensing element defining a plane;a flux guide configured to direct a magnetic field perpendicular to theplane into the plane; a plurality of reset lines configured to re-orienta magnetic field domain of the flux guide, wherein each reset line ofthe plurality of reset lines is positioned orthogonal to the flux guide;and circuitry operably connected to each reset line of the plurality ofreset lines, wherein the circuitry is configured to selectively apply acurrent to at least one reset line.
 9. A magnetic field sensor,comprising: a sensing element defining a plane; a flux guide configuredto direct a magnetic field perpendicular to the plane into the plane;and a plurality of reset lines configured to re-orient a magnetic fielddomain of the flux guide, wherein each reset line of the plurality ofreset lines is positioned orthogonal to the flux guide, wherein at leastone reset line of the plurality of reset lines is associated with a pairof switches configured to selectively apply a current to the at leastone reset line.
 10. The magnetic field sensor of claim 9, wherein thecircuitry includes a plurality of pairs of switches, and wherein eachreset line of the plurality of reset lines is associated with acorresponding pair of switches of the plurality of pairs of switches,each corresponding pair of switches configured to selectively provide acurrent to an associated reset line.
 11. The magnetic field sensor ofclaim 8, wherein, in response to a current pulse, each reset line of theplurality of reset lines is configured to generate a stabilizationfield.
 12. The magnetic field sensor of claim 10, wherein thecorresponding pairs of switches are configured to simultaneously providea current pulse to each reset line of the plurality of reset lines. 13.The magnetic field sensor of claim 10, wherein the corresponding pairsof switches are configured to sequentially provide a current pulse toeach reset line of the plurality of reset lines.
 14. The magnetic fieldsensor of claim 10, wherein the corresponding pairs of switches arefurther configured to provide a plurality of current pulsessimultaneously to two immediately adjacent reset lines of the pluralityof reset lines.
 15. The magnetic field sensor of claim 10, wherein atleast two reset lines of the plurality of reset lines are coupledtogether in series.
 16. The magnetic field sensor of claim 8, wherein atleast one reset line of the plurality reset lines is positioned at anend of the flux guide.
 17. The magnetic field sensor of claim 16,wherein the at least one reset line includes a higher spatial densitythan the other reset lines of the plurality of reset lines.
 18. Themagnetic field sensor of claim 8, wherein the flux guide comprises amaterial having a domain wall velocity of greater than 100 m/s forcurrent pulses of less than 100 ns.
 19. The magnetic field sensor ofclaim 10, wherein the corresponding pairs of switches are configured tosequentially provide a plurality of current pulses to each reset line ofthe plurality of reset lines, and wherein a final current pulse of theplurality of current pulses generates a larger magnetic field than anyof the other current pulses of the plurality of current pulses.
 20. Themagnetic field sensor of claim 8, wherein the plurality of reset linesincludes a first reset line positioned on a first side of the sensingelement, and a second reset line positioned on a second side of thesensing element opposite the first side.
 21. The magnetic field sensorof claim 8, wherein the plurality of reset lines includes first andsecond reset lines aligned with a center of the sensing element.
 22. Amethod of operating a field sensor, the method comprising: sensing, by afield sensor including a plurality of sensing elements, a magneticresponse in a direction out of a plane defined by the plurality ofsensing elements of the field sensor, each sensing element of theplurality of sensing elements having a first side and a second side, aflux guide disposed orthogonal to and spaced from the first side of thesensing element, and a plurality of reset lines positioned proximate thesensing element and adjacent to the flux guide, wherein the field sensorfurther includes control circuitry operably connected to each reset lineand configured to selectively apply at least one current pulse to eachreset line of the plurality of reset lines; selectively applying aplurality of current pulses to the plurality of reset lines; andgenerating a reset field to reset one of the field sensor and at leastone flux guide associated with a sensing element of the plurality ofsensing elements, wherein the reset field is generated as a result ofthe selectively applied plurality of current pulses.
 23. The method ofclaim 22, wherein selectively applying the plurality of current pulsesto the plurality of reset lines includes: applying a plurality ofcurrent pulses simultaneously to the plurality of reset lines.
 24. Themethod of claim 22, wherein selectively applying the plurality ofcurrent pulses to the plurality of reset lines includes: applying aplurality of current pulses sequentially to the plurality of resetlines.
 25. The method of claim 22, wherein selectively applying theplurality of current pulses to the plurality of reset lines includes:applying a first current pulse to a first reset line of the plurality ofreset lines; removing the first current pulse to the first reset line ofthe plurality of reset lines; and applying a second current pulse to asecond reset line of the plurality of reset lines after removing thefirst current pulse to the first reset line.
 26. The method of claim 22,wherein selectively applying the plurality of current pulses to theplurality of reset lines includes: applying a first current pulse to afirst reset line of the plurality of reset lines; applying a secondcurrent pulse to a second reset line of the plurality of reset lines,wherein the second reset line is immediately adjacent the first resetline; and removing the first current pulse to the first reset line,wherein the first current pulse is removed after the second currentpulse is applied to the second reset line.
 27. The method of claim 22,wherein selectively applying the plurality of current pulses to theplurality of reset lines includes: applying a current pulsesimultaneously to each of the plurality of reset lines so as tostabilize the field sensor during operation.
 28. The method of claim 22,wherein the reset field is configured to reset both the field sensor andthe at least one flux guide.